subscript

The passage around this formula

Here is the physical core. To first order, a short parallel die-to-die link is a driven capacitive wire, and the energy spent per bit is set by the capacitance that must be charged and discharged: $E_b \;\approx\; \tfrac{1}{2}\left(C_{\mathrm{fix}} + c_w \ell\right) V_{\mathrm{sw}}^{2}$. where $C_{\mathrm{fix}}$ collects driver, receiver, bump and pad capacitance, $c_w$ is capacitance per unit length of the conductor, $\ell$ is the reach, and $V_{\mathrm{sw}}$ is the signalling swing. Two assumptions are doing the work and both deserve stating. First, this treats the link as a lumped capacitance rather than a transmission line — reasonable for a few millimetres of on-package wire at moderate rates, wrong once the link is long or fast enough to need equalisation,…

Read this part in the article →